The Difference Between Function and Block Scope in JavaScript

Back to the basics with the var, let and const variables

The difference between var, let and const variables

There are three ways to declare variables in JS:

var width = 100;
let height = 200;
const key = 'abc123';

var

What is unique about var? It can be reassigned and updated. For example:

// Define the variable:
var width = 100;
// Call the variable:
width;
// It returns:
100
// Reassign the variable and call it again:
width = 200;

width;
// Returns:
200
function setWidth(){
var width = 100;
console.log(width);
}
width;
// Returns:
Uncaught ReferenceError: width is not defined

Understanding scope

Example: var age = 100;
if (age > 12){
var dogYears = age * 7;
console.log(`You are ${dogYears} dog years old!`);
}
100
You are 700 dog years old!
dogYears;
// returns:
700

Benefits of using let and const

What are the benefits of using let and const? Rather than being scoped to the function they are scoped to the block.

var age = 100;
if (age > 12){
let dogYears = age * 7;
console.log(`You are ${dogYears} dog years old!`);
}

Recap

var is function scope.
let and const are block scope.
Function scope is within the function.
Block scope is within curly brackets.

Coding, creativity, music, and books. Pianist & composer — @vontmer

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store